As is known, an anti-fouling paint is one which is intended to be applied to the hulls of ships or to the surface of any other immersed object, usually on to a coating of rust-resistant paint, and the purpose of which is to prevent or minimise the fixation of certain marine flora and fauna, for example algae, shellfish, mulluses and corals on to the hull or surface.
Several types of anti-fouling paints are known.
Certain anti-fouling paints operate by exfoliation, i.e. by flaking from the substrate and thereby carrying away the marine organisms attached to the surface of the paint.
However, most of the anti-fouling paints operate by progressive liberation of toxic substances, the purpose of which is to destroy the marine organisms coming into contact with the hull or at least to inhibit the fixation thereof on the hull.
In this last type of anti-fouling paint, the liberation of the toxic substance results from the leaching out or release of this substance by the sea water, this being a phenomenon which is produced just as well when the ship is moving as when it is stationary. Certain toxic pigments, such as cuprous oxide, even have a higher leaching out speed when the ship is in motion than when it is stationary.
Now the tendency for the fauling media to become fixed only exists in practice when the hull is stationary in relation to the surrounding mass of water. It is thus pointless for the toxic substance to be liberated while the ship is moving, sine this results in a waste of the toxic substance and a premature loss of efficiency of the anti-fouling paint.
With the object of prolonging the effective duration of an anti-fouling paint, it has been proposed in French Pat. No. 2,155,166 to seal the toxic product inside microcapsules which have a water-soluble casing formed by a substance selected from the group comprising gelatine, nitrocellulose and methylcellulose, the diameter of the capsule being from 5 to 30microns and the thickness of its casing being from 3 to 10 microns, these microcapsules being dispersed in a proportion which is between 5 and 25% in the binding agent or vehicle of the paint.
According to this patent, the liberation of the toxic product as thus sealed in the microcapsule can take place by various procedures, namely, either by osmosis through the wall, or by dissolution of the said wall, or by bursting of the wall under the action of a blowing agent or under the mechanical action produced by brushing.
In order to regulate the release of the toxic product in time, thereby stabilising its diffusion, it is mentioned in the same patent that it is possible for the toxic agent to be incorporated into the microcapsule in admixture with a resin having the property of polymerising under the action of humidity.
The process described in the aformentioned patent enables the speed of release of the toxic substance to be slowed down or regulated to a certain degree, but it does not however prevent entirely the leaching out of the substance while the ship is under way.